In a transmitter for transmitting a radio signal from an antenna, in order to detect a case where an abnormality, such as a failure or unconnected state of the antenna occurs in the antenna, a technique has been proposed which measures the reflection power reflected from the antenna and which detects the abnormality on the basis of the magnitude of the measured reflection power (see, for example, Patent Literature 1).
FIG. 1 is a diagram showing one form of a radio apparatus using a common power detection circuit.
The radio apparatus shown in FIG. 1 includes transmission signal output section 1000, power detection circuit 10, and antenna 4000. Transmission signal output section 1000 is an amplifier which outputs to antenna 4000 a transmission signal to cause a radio signal to be transmitted from antenna 4000.
Power detection circuit 10 detects the reflection power of the reflection signal reflected from antenna 4000.
Antenna 4000 transmits, as a radio signal, the transmission signal outputted from transmission signal output section 1000.
Further, power detection circuit 10 is configured by reflection power detection section 3000 and comparison section 7000. Further, reflection power detection section 3000 is configured by coupler 3001 and detector 3002.
Coupler 3001 branches the reflection signal reflected from antenna 4000, so as to output the branched signal to detector 3002.
Detector 3002 converts to a DC voltage the reflection power of the reflection signal outputted from coupler 3001. Further, detector 3002 outputs the converted voltage to comparison section 7000.
Comparison section 7000 compares the voltage outputted from detector 3002 with a preset threshold value. Further, when the comparison result is one in which the voltage outputted from detector 3002 is larger than the preset reference value (threshold value), comparison section 7000 outputs an alarm (ALM).
In the radio apparatus configured in this way, when the return loss (reflection power) from antenna 4000 is increased, the reflection power taken out (branched) by coupler 3001 is increased. The reflection power is detected by detector 3002, and then, in comparison section 7000, the detected reflection power is compared with the reference value to determine whether or not an alarm is to be issued.
Here, the reflection power is proportional to the transmission power of the transmission signal outputted from transmission signal output section 1000. Therefore, when the transmission power is small, it is not possible to determine the abnormality of antenna 4000. In the case where the transmission power is too large, even when the return loss (reflection power) of antenna 4000 is in a normal range, there is a case where, since the absolute value of the reflection power is large, the state of antenna 4000 is determined to be abnormal.
In this way, since an accurate determination cannot be made according to the change in the transmission power, there is a problem that, when the signal level is successively changed as in the case of a burst signal, a correct determination cannot he made.
Thus, in addition to the configuration shown in FIG. 1, a technique has been proposed in which the transmission power transmitted from the amplifier to the antenna is also detected so that the abnormality is detected on the basis of the difference between the detected transmission power and the reflection power (see, for example, Patent Literature 2).